MgB2 superconductor has a high critical temperature Tc compared to practically used superconducting materials, and also has advantages such as follows in view of practical use.
i) On the occasion of passing a large superconducting current from one crystal grain to a neighboring crystal grain, it is considered that alignment (orientation) of crystal grains such as in the case of high temperature oxide superconductors is unnecessary;
ii) the material is abundant in resources, and the raw materials are relatively inexpensive;
iii) the material is mechanically tough; and
iv) the material is lightweight.
Accordingly, MgB2 superconductor is considered to be promising as a practically useful material, and research and development on this material is currently in progress.
On the other hand, for the practicalization of superconducting materials, so-called wire fabrication must be achieved; however, regarding the method for fabricating MgB2 wire, a so-called powder-in-tube (PIT) method in which a metal tube is packed with a raw material powder, the raw material powder is processed into wire, and then the wire is heat treated, is most generally used (see Patent Literature 1). However, MgB2 superconducting wire produced by the PIT method has a serious problem that the critical current density Jc, which is the most important for practical use, is far below the level for practical use.
For this reason, various attempts have been made to enhance the Jc characteristics through an increase in the packing density, addition of impurities, and the like (see Non Patent Literatures 1 to 3). Among these, in regard to the addition of impurities, addition of impurities including carbon has been hitherto attempted many times. The most common additive is SiC nanoparticles, and it has been reported that the Jc characteristics in high magnetic fields are significantly enhanced through the addition of SiC nanoparticles. This is speculated to be because the addition of SiC induces substitution of some sites of B in MgB2 crystals with C, and thereby, the upper critical magnetic field Hc2 is increased. However, there is a problem with the addition of SiC that Mg2Si remains as an impurity after a heat treatment, and a sufficiently high value of Jc is not obtained.
On the other hand, substitution of the sites of B with C also occurs as a result of addition of an aromatic hydrocarbon or the like, and enhancement of the Jc characteristics in a high magnetic field may be attained by a similar mechanism (see Patent Literatures 1 to 3). Then, since there is no remaining of impurities such as Mg2Si in the addition of an aromatic hydrocarbon, it is considered that it is therefore more advantageous in the increase of Jc. In a case in which the aromatic hydrocarbon is a liquid, the liquid covers the surfaces of B or the Mg particles, and thus a uniform mixture is obtained. However, in a case in which the aromatic hydrocarbon is a solid, with regard to the addition of this solid, mixing of an Mg+B mixed powder and a solid aromatic hydrocarbon is not necessarily achieved uniformly, even if a ball mill or the like is used. Therefore, there is a problem that it is difficult to obtain wire having less fluctuation in the Jc characteristics.